Brassicaceae plants include many plants whose entire plant bodies are edible, e.g., oilseed rape, turnip, Japanese mustard spinach, Chinese cabbage, bok choy, and Japanese radish, and for example, Brassica rapa includes a plant producing seeds having high utility value as a material for rapeseed oil. Conventionally, rapeseed oil has been used for food or lighting fuel, and in recent years, it receives attention as a material for biodiesel.
Currently, for production of rapeseed oil, Brassica napus is mainly used. Brassica napus is actively cultured in North America and Canada because it has self-compatibility and excellent seed productivity, however, growth and production efficiency thereof in other area is not good, and breed improvement is required. However, since Brassica napus is believed to be an amphidiploid species generated by natural crossbreeding between Brassica rapa and cabbages (Brassica oleracea), and has poor genetic diversity, it has been difficult to apply a conventional breeding method that introduces a beneficial gene by crossbreeding.
On the other hand, in Japan, Brassica rapa has been traditionally used as a material for rapeseed oil. Brassica rapa has immense genetic diversity, and has the potential of making advantageous strains in terms of the environmental suitability, improvement in seed productivity and so on. However, since Brassica rapa is self-incompatible and does not produce a seed in a single line, it is necessary to keep the lineage relying on natural crossbreeding by insects or the like in cooperative cultivation, and hence it is difficult to keep a specific lineage stably. Further, the seed productivity is low because crossbreeding with other individual is required to produce a seed, and breed improvement should be made on the whole mass to be crossbred, and hence it was difficult to breed an excellent lineage.
From such a background, there is a demand for a technique of converting self-incompatible Brassicaceae plants including Brassica rapa to ones having self-compatibility to thereby impart self seed fertility, and enabling efficient making and keeping of an excellent breed by utilizing the genetic diversity.
Here, self-incompatibility is one of the functions of suppressing self-pollination possessed by plants. This function causes self-other recognition between a pistil and pollen at the time of pollination, and allows pollination of only pollen of other individual. That is, in a plant having self-incompatibility, a seed is not formed when a pistil and pollen have the same genotype, because even if the pollen reaches the stigma, either of the stages including germination of pollen, growth of pollen tube, fertilization of ovule, and growth of fertilized embryo terminates. In many plants, such a function of self-incompatibility is controlled by a series of multiple allele cluster (S1, S2, . . . Sn haplotype) linked on a self-incompatibility gene locus (S gene locus). In a Brassicaceae plant, S haplotype encodes pollen factor SP11 which is to be a ligand, and a pistil factor SRK functioning as a receptor, and by specific interaction between SP11 and SRK on the same S gene, the own pollen is discriminated, and incompatible reaction occurs. Further, it is also known that stigma protein (SLG: S locus glycoprotein) exists on the S gene locus, which has a nucleotide sequence very similar to that of SRK protein, and is believed to function as a common receptor and extend the self-incompatibility.
Over 100 S haplotypes are known in Brassicaceae plants, and relation of dominance sometimes arises between two haplotypes. It has been revealed that in such a case, by the influence of an inverted transcription sequence (SMI) on the dominant S haplotype classified into class I, a SP11 expression regulation region on the recessive S haplotype classified into class II is DNA-methylated, and expression of the recessive S haplotype is completely suppressed (See, for example, Non-Patent Documents 1 and 2, and FIG. 1).
As described above, while various findings regarding the mechanism of self-incompatibility have been obtained, a practicable and convenient technique for efficiently converting a self-incompatible Brassicaceae plant to one having self-compatibility is still unknown.